It is important for the interfacial surface of expandable tire curing bladders of a butyl rubber composition to have adequate interfacial lubricity and sufficiently low adhesion properties between the bladder surface and the inner surface of the tire being vulcanized.
Conventionally the butyl rubber composition for a tire curing bladder contains castor oil as a lubricant which tends to bleed to the outer surface of the bladder to promote a continuing lubricity at the interface between the bladder surface and inner surface of the tire being cured. For example, see U.S. Pat. No. 3,031,423.
Use of corn oil as an internal tire cure butyl rubber bladder lubricant has also been proposed. See U.S. Pat. No. 5,580,513.
Use of graphite as a lubricant (see U.S. Pat. No. 5,538,218) and polytetrafluoroethylene powder as a lubricant (see U.S. Pat. No. 5,728,311) have also been proposed for use in a butyl rubber tire curing bladder.
Even with an internal lubricant, such as castor oil or corn oil for the butyl rubber bladder composition, it is often desired to apply additional lubricant to the outer bladder surface to enhance the aforesaid interfacial lubrication such as, for example, a polysiloxane-based lubricant, a practice which is well known to those skilled in such art.
From an historical perspective, pneumatic rubber vehicle tires are produced by molding and curing a green or uncured, and unshaped tire in a molding press. The green tire is pressed outwardly against a mold surface by means of an inner fluid-expandable bladder. By this method, the green tire is shaped against the outer mold surface which defines the tire tread pattern and configuration of the sidewalls. By application of heat and pressure, the tire is molded and cured at elevated temperatures.
Historically, the expansion of the bladder is accomplished by application of internal pressure to the inner bladder cavity which is provided by a fluid such as gas, hot water and/or steam which also participates in the transfer of heat for the curing or vulcanization of the tire. The tire is then conventionally allowed to cool somewhat in the mold, sometimes aided by adding cold or cooler water to the bladder. Then the mold is opened, the bladder is collapsed by removal of its internal fluid pressure and the tire is removed from the tire mold. Such use of tire curing bladders is well known to those having skill in such art.
By such practice, it is recognized that there is substantial relative movement at the interface between the outer contacting surface of the bladder and the inner surface of the tire during the expansion phase of the bladder. Likewise, there is considerable relative movement between the outer contacting surface of the bladder and the cured inner surface of the tire during the collapse and the stripping of the bladder from the tire after the tire has been molded and vulcanized.
By such practice, it is recognized that the bladder surface can tend to stick to a tire's inner surface after the tire is cured and during the bladder collapsing part of the tire cure cycle. This adhesion may cause roughening of the bladder surface if it is not controlled. This typically reduces bladder durability and can produce defective tires.
Accordingly, it is often desired to pre-coat the bladder surface, or to pre-coat the inner surface of the green or uncured tires with a lubricant which can also transfer to the bladder surface and, thereby, provide a degree of interfacial lubricity between the outer bladder surface and inner tire surfaces during the tire's molding and curing operation. Such lubricant has sometimes been referred to as a "bladder lubricant", and can be of numerous formulations. A silicone polymer (e.g.: a polysiloxane) is often used as a bladder lubricant.
It is to be appreciated that the release of the tire from its curing bladder in an industrial manufacturing setting is intimately associated with both the phenomenon of release (to prevent sticking) and the phenomenon of lubrication (to enhance slipping) between the bladder and the adjacent tire surfaces. The release aspect refers to the basic ability to avoid adhesion, or release, and the aspect of lubrication relates to enhancing the ability of the surfaces to slip and enable a movement of the bladder with respect to the tire.
Butyl rubber is commonly used in tire curing bladders. Butyl rubber is a copolymer of predominantly isobutylene with small amounts of diene monomers usually isoprene to give sufficient unsaturation to allow the butyl rubber to be crosslinked. Halogenated copolymers of isobutylene and para-methylstyrene are also sometimes used.
Accordingly, it is desired to provide curing bladders with acceptable lubricity, decreased adhesion of the bladder to cured tires, increased flex life, and improved heat stability.
In the description of this invention the term "phr" is sometimes used to refer to "parts per hundred parts by weight of rubber" for various ingredients in a rubber composition.
The terms "compound", "compounded rubber" and "rubber composition" are intended to be interchangeable terms unless otherwise noted.
The terms "cure" and "vulcanize" are intended to be interchangeable terms unless otherwise noted.